A lamp which is representatively a halogen lamp and generates infrared rays, or a method of performing heating with Joule's heating by using electromagnetic induction is put into practical use as a heat source of a fixing device which is mounted in an image forming apparatus.
Generally, the fixing device is configured by a pair consisting of a heating roller (or a fixation belt crossing over a plurality of rollers) and a pressing roller. However, it is required that the heat capacity of components is reduced as much as possible and heating is performed focused on a heating area, in order to maximize the thermal efficiency of the fixing device. In this regard, in the above-described heating method, the width of a heating area is wide and thus it is difficult to apply heat energy which is dispersed in a wide range to only a nip portion intensively and it is difficult to optimize the thermal efficiency.
In a fixing device for electrophotography, when heating unevenness occurs in a perpendicular direction to a paper transporting direction, the unevenness has an influence on fixing quality. Particularly, when color printing is performed, a difference in color formation or luster may occur.
In a fixing device having extremely reduced heat capacity, the temperature at a portion through which paper does not pass is extremely increased. Thus, a problem such as speed irregularity may occur due to warpage of a heater, deterioration of a belt, and expansion of a transporting roller. In view of energy saving, heating the portion through which the paper does not pass is not preferable. In view of environmental correspondence, intensively heating only a portion through which paper passes causes an important technical problem.
An example of the related art includes JP-A-2000-243537.